Aluminide coatings modified with electroless platinum plating for turbine blade protection

Modification of aluminide coatings by addition of a Pt layer improves their high temperature oxidation resistance considerably. The layer is deposited prior to aluminizing and optimal resistance capabilities are obtained with 5-8 μm thickness. The only method of platinum deposition currently employed at industrial scale is electroplating, which involves several drawbacks when it comes to industrial plating of sharp geometries, such as turbine blades. For this reason, this work takes the challenge of developing a simple and stable electroless solution for platinum plating on Ni-based superalloy, capable of depositing coatings with thickness that complies with the practical necessities of industry. The proposed plating solution is based on hydrazine monohydrate as the reducing agent in acidic environment. Purity of the electroless coating was confirmed by EDS and XRD analysis. Growth mechanism was evaluated by SEM imaging at different stages of deposition and it was found that nucleation of platinum nanoparticles starts in correspondence of catalytic sites with high surface energy and growth occurs with a high degree of internal stresses that lead to cracking . Thickness was evaluated by weight gain measurements and observations by SEM in cross section. Deposition properties were assessed investigating the effect of several parameters (temperature, pH and bath composition) on plating rate and morphology. Best parameters were selected for a 3h deposition. Eventually, the effect of a pre-aluminizing (or prior) diffusion heat treatment on the evolution of Pt-aluminide coatings on René N4 Ni-based superalloy was evaluated using a high temperature low activity aluminizing process. Microstructure of Pt-modified coatings formed adopting a diffusion schedule at 950°C for 6 hours was then evaluated and compared to that generated without any prior diffusion treatment.